1) Enterprise PC sales & upgrades

Many corporates have already announced their intention to steer clear of Windows 8 upgrades, as the new UI would bring additional training costs along with it. In addition, they don't see the need to upgrade to an operating system that does not bring too many other benefits with it to their existing hardware (Storage Spaces, Reset and Refresh are not enough to compensate for the new Desktop without the start button and the Metro "Start Screen").

Regarding enterprise PC sales, new PCs may still come with Windows 8 pre-installed, but these sales have been reducing as corporates put off hardware upgrades in favor of acquiring tablets. Noticing this trend, companies like IBM have even begun offering leases on iPads and Android Tablets. Windows 8 would do nothing to slow this trend.

2) Consumer PC sales & upgrades

Again, very few consumers would upgrade to Windows 8 as the new UI offers no real benefits to existing hardware.

It was only in late 2011 that Windows 7 overtook Windows XP as the most used version of Windows. Clearly the upgrade cycles for Windows upgrades are considerably long. And by the time Windows 8 gets its turn, the PC market may not exist in its current form anymore, as PC sales have been cannibalized by increased spending on smartphones and tablets.

Another factor to consider is the launch of Ultrabooks with Windows 8. Now, Ultrabooks have been in the market for a while, but sales have been disappointing due to high prices. These high prices have been driven by large SSD hard drives and x86 hardware optimized for the compact form factor. Now with Windows 8, ultrabooks would ideally need to incorporate touch input as well, which would drive prices even higher, with no chance of truly making an impact on the market.

3) Tablets with x86 Architecture

This is an easy one. In the future, this may overlap with hybrid ultrabook form factors (think Asus Transformer
& Transformer Prime). If ultrabooks are too expensive, x86 tablets would be even more expensive and priced out of the market. Even though these tablets would be capable of running legacy windows applications (with shoddy touch support) and have full office functionality, if consumers aren't buying an ultrabook for $800, they sure as hell aren't buying a tablet for $1000.

4) Tablets with ARM Architecture

Now, this is where Microsoft and a lot of analysts feel the growth of Windows 8 is going to come from. ARM hardware is cost effective and, with the launch of nVidia's Tegra 3 processor, offers processing power comparable to a PC, with console quality graphics. If these tablets are priced similar to most Android tablets, and below the iPad, why wouldn't they sell? It's Windows after all, right?

Wrong! Windows on ARM (or Windows RT) does not offer legacy application support - Meaning, none of your windows programs, games or hardware accessories will work with these devices. The only applications that can run on it will be metro applications, from the yet to be launched, windows store (which works with x86 tablets as well). It will come with a "mobile optimized version of office", so a consumer's choice is between Apple/Android tablets with a whole range of apps to choose from and semi-functional office to a blank Windows RT tablet with Office. Considering these are content consumption devices, the choice is obvious.

For content consumption, app selection is one of the most important criteria. App selection is driven by number of developers. Number of developers is driven by app revenue. App revenue is driven by sales volume. Sales volume is driven by usability. Usability is driven by..... you guessed it, app selection. This chicken & egg situation is caused by Microsoft's late entry into this market, which I think will be the downfall of Windows 8 (the same explanation is valid for Windows Phone as well).